Thoughts on tyre drag.

[joel0407]

Well I'm just thinking about this.

Usually when we put bigger tyres on, our fuel consumption goes up. The question, is this because the tread create more drag, is it because our gearing changes or is it the rotational mass taking more power to turn?

In theory the larger diameter wheel should mean a higher ratio (gearing) so we would effectivly gear up. This should get better fuel consumption on the open road becasue our engine revs will drop but maybe increasing the engine load is increasing fuel consumption. Then maybe by changing out diff or tranfer box ratios we would maintain fuel consuption.

In theory the larger wheels would have a lesser flex angle on the road so less flexing of the tread would mean less heat, longer tyre life, less resistance and better fuel consumption.

In theory by fitting wider tyres the contact patch area doesnt change but the contact patch goes from long and skinny to short and wide. What is gained by the contact patch being shorter, meaning less flex angle is offset by the flexing being wider. All this means the same fuel consumption.

One possiblilty is bigger tyres carry more load so we need to run them at a lower pressure and this will then increase contact area, increase flex angle and therefore resistance. All increasing drag and fuel consumption.

One big misconception of wider tyres is they give more grip. That may or may not be true off road as tread paterns come into play. On road, as the area increases the pressure per square reduces and therefore while the contact area is larger the friction provided by each square of that larger area is less, the end friction provided by the tyre is the same.

Race cars use wider tyres but they gain the advantage of longer life and more durability which means they car run a softer compound and acheive the same tyre life. In normal road vehicles by fitting wider tyres we actually get less grip because the tyres will heat up less which means the rubber will stay harder and provide less grip but they will last longer. Not mentioning aquaplanning as I think most will be aware of this.

Maybe it's the wind resistance of the larger tyres that causes the extra fuel consumption.

All this thought has come about because I just like the look of bigger tyres. Not too big, nothing like 37" or anything but about 32" is a good look on a 4x4 just my personal preferance and as I said about the bigger size will last longer. But I don't want the extra fuel consumption so if I change my transfer box ratio to maintain the original gearing will my fuel consuption stay the same.

Happy Days.

[Scouse]

While I'm sure all your theories will have some impact on fuel consumption, the main factor is the gearing difference.


With your average Land Rover, the engine will be working harder to maintain the speed with the higher gearing.
You might find some economy gains on nice flat freeways though.

[Dougal]

Tread pattern influence is huge.
Go ride a bike with slick tyres and then with low pressure knobblies.

Easiest way to measure tyre drag change is to measure your coast down times and then again with Chunkier tyres.

[joel0407]

That's interesting Dougal. Using the coast down test you could measure actual tread drag without the consideration of gearing and wind resistance would reduce as the vehicles slowed.

That would prove or disprove bigger tyres might roll easier but it would still leave the question is it the greater wind resistance or the gearing that causing the higher fuel cost.

I see how the higher gearing creating more enging load could cause more fuel consumption but I doubt it a little. The ratio change is not usually that great, maybe 10%. When we consider that most vehicles actually improve fuel consumption with higher ratios like the differance in driving in 4th or 5th gears. The differance in ratio for 4th and 5th gears are far more than the ratio change in up sizing tyres.

Some could argue the vehcile looses ratio and power for acceleration as well but that should only really effect first part of 1st gear from then on you will actually be in a lower gear for a given speed as you would hold a gear a little longer. For example if you previously changed to 2nd at 30km/h with the higher ratio provided by the bigger tyres, you'd change to 2nd at 35km/h. Meaning while the engine gets a little more load through most of the rev range, it gets it a little bit easier for the last part of the rev range.

And to top off the gear ratio thing. Most recommendations for driving economically say to short shift and rev the motor less to save fuel which indicates to me an engine will use less fuel with low revs and heavy load than high reves and low load. I know there is a bit of lose in engine friction but is this enough to acount for the extra fuel from bigger tyres.

Happy Days.

[JDNSW]

In my view, the major factors that affect rolling losses and hence fuel consumption are:-
1. the energy converted to heat in flexing the tyre - which almost entirely depends on the construction of the tyre and the rubber compound used, but is pretty much independent of tyre size.

2. Energy converted to heat and sound by the tread - which again, depends entirely on the tread pattern and composition, hence is independent of tyre size, at least within the bounds of practical sizes.

Tyre diameter affects effective gearing, but for most modern vehicles I would be surprised if it made much difference to fuel consumption, again within the limits of practical sizes. Same applies to the effects of aerodynamic drag, although this is not a simple calculation, as the turbulent airflow around the tyre will be affected by the proximity of the bodywork.

Tyre pressure is of course a major factor in fuel consumption, and another one, not often recognised, is that tyre tread width also affects consumption on anything except a straight road, as on a corner the edges of the tyre are necessarily either skidded on the road or the tread flexes (depending on the tyre properties and the surface), either process converting fuel energy to heat.

John

[joel0407]

I'm not surprise the first 2 responses to the thread, one says drag from the tyre and the other says gearing.

[joel0407]

1. the energy converted to heat in flexing the tyre - which almost entirely depends on the construction of the tyre and the rubber compound used, but is pretty much independent of tyre size.




I have to disagree that it's independant of tyre size. As the diameter increases the angle the tread meets the road is reduced as such so is the flex of the tyre. So going up in size but sticking with the same model of tyre should reduce fuel consumption.



2. Energy converted to heat and sound by the tread - which again, depends entirely on the tread pattern and composition, hence is independent of tyre size, at least within the bounds of practical sizes.


Again so if sticking with the same brand and model of tyre. Fuel consumption should stay the same at least.


and another one, not often recognised, is that tyre tread width also affects consumption on anything except a straight road, as on a corner the edges of the tyre are necessarily either skidded on the road or the tread flexes (depending on the tyre properties and the surface), either process converting fuel energy to heat.

John

Here in the NT most roads are straight and I'd wonder how much time our vehicles actually spend turning.

I presume you are taking about slip angle?

_______________________________________

Everything points to bigger tyres being more fuel efficent but we all know that's just not the case so why?

Happy Days

[joel0407]

I've been searching and reading for a while now and I am begining to be convinced that larger tyres will have less drag and going up in size has a far bigger effect on rolling resistance (reduced) than increasing tyre pressure.

Partly the reason fuel consumption goes up is down to the extra energy required to spin the heavier wheel but this is not relevant once desired speed is reached.

The main reason is wind resistance. Partly from the extra width of the tyre but grossly from lifting the vehicle up even only a small amount will greatly effect aero dynamics of the vehicle, create more wind drag and ultimatly cost more fuel.

I know from previous personal experience that lowering a vehicle, even slightly will reduce fuel consumption. I had a WRX that I lowered only 1/2" from standard and saved on average 0.5L/100km. I drove from Canberra to Bathurst plenty of times and it was immediatly obvious in my fuel consumption when I changed the suspension.

Still open to further comments and ideas.

Happy Days.

[Naviguesser]

How are you calculating the distance traveled to get your figures.
Don't forget the odometer will read a different distance with the different tyres fitted

[joel0407]

This is interesting.

tire width vs. drag (Cd) - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com (http://ecomodder.com/forum/showthread.php/tire-width-vs-drag-cd-7475.html)

The Honda CRX had a 9.3% increase in drag going from 165 to 185 tyres.:eek:

Happy Days

[joel0407]

How are you calculating the distance traveled to get your figures.
Don't forget the odometer will read a different distance with the different tyres fitted

Yeah, I'm aware of that. As most speedometers read over X% as standard the change to bigger wheels and reading Y% over means the change is X% + Y% not just Y% alone. That can make a considerable differance.

My Skoda Yeti reads at least 5% over actual road speed so even if I use the Odometer to calculate my economy, I haven't actually covered the distance it's reading. These days you can just have the ECU calibrated to correct it but it's a bit of a guessing game with the calibration as the numbered in the Skoda/VW/Audi ECU don't actually relate to anything. You just change it be a random number then see the differance. Once you work out what each unit is worth then you know how mnay to change it by but it's trail and error to work it out. I would probably do it if I had someone with a computer to access my ECU close enough who didn't charge a fortune. It's a bit hard in Darwin.

Happy Days

[nismine01]

I used to work for a Ford dealership. we had a customer who had two Mavericks (Nissan Patrol), they constantly complained that one of them had less power when pulling a large boat, on it's trailer (just in case anyone wanted to suggest that would make a difference). It also used more fuel, both diesels. Several tests were done but nothing could be found, one day I serviced one of them next day I serviced the other, HAD IT.
Told the boss I had the answer, one had a wide wheel pack.
Got them both in next day, swapped the wheels over, I drove the 'slow one', the boss followed in the 'quick one', the slow one ran away, wider, thus larger as well was over gearing the vehicle.
We had a side by side example of what happens wit a larger tyre on the vehicle, theorise all you like.

Mike

[joel0407]

I used to work for a Ford dealership. we had a customer who had two Mavericks (Nissan Patrol), they constantly complained that one of them had less power when pulling a large boat, on it's trailer (just in case anyone wanted to suggest that would make a difference). It also used more fuel, both diesels. Several tests were done but nothing could be found, one day I serviced one of them next day I serviced the other, HAD IT.
Told the boss I had the answer, one had a wide wheel pack.
Got them both in next day, swapped the wheels over, I drove the 'slow one', the boss followed in the 'quick one', the slow one ran away, wider, thus larger as well was over gearing the vehicle.
We had a side by side example of what happens wit a larger tyre on the vehicle, theorise all you like.

Mike

Nothing unusual there, mate. We know what happens but the question is why. In theory they should use less fuel but in practice they use more. Bigger wheels will gear a vehilce up and make acceleration slower and when towing I'd expect it to use more fuel.

If gearing was all there is to it then by changing diffs or transfer box gearing, a vehicle should use the same or less. I'd like to fit True-Trac diffs to the Disco and bigger wheels but I don't want to run at 15L/100km so maybe I'll weigh up the cost of transfer ratio change or diff ratio change while I have the centres out. But things I have been reading are indicating the extra drag is from air drag on the entire vehicle, not tyre drag as tyre drag should be reduced.

Happy Days

[85 county]

This is interesting.

tire width vs. drag (Cd) - Fuel Economy, Hypermiling, EcoModding News and Forum - EcoModder.com (http://ecomodder.com/forum/showthread.php/tire-width-vs-drag-cd-7475.html)

The Honda CRX had a 9.3% increase in drag going from 165 to 185 tyres.:eek:

Happy Days

well it is about a 10% increase in with.

[85 county]

Well I'm just thinking about this.

Usually when we put bigger tyres on, our fuel consumption goes up. The question, is this because the tread create more drag, is it because our gearing changes or is it the rotational mass taking more power to turn?

All of the above sometimes

In theory the larger diameter wheel should mean a higher ratio (gearing) so we would effectively gear up. This should get better fuel consumption on the open road because our engine revs will drop but maybe increasing the engine load is increasing fuel

Correct, it all depends on the most efficient range load of your motor, and wind drag. IE a 110 starts chewing the gas over say 95 klms, larger dia wheels will be come a smaller % of overall consumption. In short larger rolling dia may or may not increase consumption


[QUOTE=joel0407;2209833]consumption. Then maybe by changing out diff or tranfer box ratios we would maintain fuel consumption.

As above, it may improve it may not. Depends on a lot of factors.

In theory the larger wheels would have a lesser flex angle on the road so less flexing of the tread would mean less heat, longer tyre life, less resistance and better fuel consumption.

Dodgy logic. Heat vers traction on road tyres is a small factor, Block distortion and air pressure are by far greater factors.

1 over dia will load up the motor, increase fuel consumption.
2 Under dia, over revving the motor moves it out of its best rev range
3 Wheel weight, or unsprung weight, has similar effects as bad shocks, worse consumption.
4 Wider tyres, may or may not increase decrees traction, depends on pounds per square inch and actual optimum compound
5 Wider tyres, increasing the foot print will increase fuel consumption
6 Low tyre pressures will increase foot print, increase fuel consumption, see No5
7 Lower tyre pressures, reduce rolling dia, may or may not affect fuel consumption, but is a mute point since No6 will out weight it in effect.

Best fuel consumption tyres are skinny as hell bald and pumped up to like 200000000 psi

Worse fuel consumption is soft compound, 8ft wide and running 0.00003 psi

The ideal is some where in the middle


Race cars use wider tyres but they gain the advantage of longer life and more durability which means they car run a softer compound and acheive the same tyre life.

Nope sort off. Race cars work on a forulur. Distance, grip weight. = compound with weight. And heat of the day loading etc


In normal road vehicles by fitting wider tyres we actually get less grip because the tyres will heat up less which means the rubber will stay harder and provide less grip but they will last longer. Not mentioning aquaplaning as I think most will be aware of this.

Nope
Forget about heat in road rubber, it’s a small issue. Manufactures spend a lot on R&D to get rid of heat. Wider tyres = less grip only if the foot print is over the ideal pounds per square inch for the compound. So bigger is not always worse. Way to big is just as bad as two small

Maybe it's the wind resistance of the larger tyres that causes the extra fuel consumption.

Yes if you have exposed wheels. Like a F1 car. See the elfin? cars of the 70s
You also need to look at the relationship between body and wheels. Big topic minor effect

All this thought has come about because I just like the look of bigger tyres. Not too big, nothing like 37" or anything but about 32" is a good look on a 4x4 just my personal preference and as I said about the bigger size will last longer. But I don't want the extra fuel consumption so if I change my transfer box ratio to maintain the original gearing will my fuel consumption stay the same.

You have a d2 TD5, and you want to run 32” rubber. You are in the NT so mud can be an issue.

Ok assuming you are going to run with a mud. The difference between 29” mud and 32” is minimal. If you were changing to 32 from 29 mud, probably 20-30 klm a tank. If you are changing from 29 road rubber maybe 60 klms a tank

29 X 3.145 = 91
32 X 3.145 = 100

9% so I would say given the same with, wind drag on the body is the biggest consumer of your fuel

[joel0407]

well it is about a 10% increase in with.



If you read the link, it's actually 9.3% for the entire cars drag. Not just the drag of the tyre.


Happy Days

[Slunnie]

My experiences rather than theory.

When I changed from 29" Michelin XPC4X4 (road) to 33" Cooper ST (AT) my consumption in town worsened by about 1l/100km and my consumption on the hwy improved by about the same. Consumption also became variable where it was previously quite consistent.

When I changed from the 33" Cooper ST to the 33" STT (mud) consumption worsened everywhere by I think it was about 1l/100km.

When I changed from the 33" STT to the 33" Simex JT2 (Bog) consumption worsened everywhere by another 1l/100km.

As a note, when I corrected the gearing for the 33" tyres by changing from 3.54:1 to 4.11:1 the consumption became fairly consistent again.

[joel0407]

My experiences rather than theory.


As a note, when I corrected the gearing for the 33" tyres by changing from 3.54:1 to 4.11:1 the consumption became fairly consistent again.

Did your fuel consumption get any better or just more consistent?

What size were/are your 33" tyres and are they wider than the 29" tyre? I'm guessing that if they were not that much wider the extra wind drag was offset by the better (less) rolling resistance of the larger diameter tyre.

Happy Days

[Hoges]

FWIW...From various tyre company and other websites, changing from road tyres (H/T) to A/T can expect 15% increase in fuel consumption. My experience changing from Cooper h/t 255/65/16 to Bridgestone dueler 245/70/16 was significant increase in fuel on highway use and slow down on coasting is much quicker withthe A/Ts due to drag... not aerodynamic but rather friction drag due to the very different tread pattern...

[joel0407]

Well I'm just thinking about this.

Usually when we put bigger tyres on, our fuel consumption goes up. The question, is this because the tread create more drag, is it because our gearing changes or is it the rotational mass taking more power to turn?

All of the above sometimes

In theory the larger diameter wheel should mean a higher ratio (gearing) so we would effectively gear up. This should get better fuel consumption on the open road because our engine revs will drop but maybe increasing the engine load is increasing fuel

Correct, it all depends on the most efficient range load of your motor, and wind drag. IE a 110 starts chewing the gas over say 95 klms, larger dia wheels will be come a smaller % of overall consumption. In short larger rolling dia may or may not increase consumption


[QUOTE=joel0407;2209833]consumption. Then maybe by changing out diff or tranfer box ratios we would maintain fuel consumption.

As above, it may improve it may not. Depends on a lot of factors.

In theory the larger wheels would have a lesser flex angle on the road so less flexing of the tread would mean less heat, longer tyre life, less resistance and better fuel consumption.

Dodgy logic. Heat vers traction on road tyres is a small factor, Block distortion and air pressure are by far greater factors. If you run a heavy loaded vehicle with tyre monitors you soon learn that heat is a big issue in normal rd tyres. A larger tyre will require less pressure for the same weight and dissipate heat better. Rubber will literally fly off a normal road tyre if it over heats. I've also seen normal road tyres get sticky with surface of the rubber looking just like hot racing slicks.

1 over dia will load up the motor, increase fuel consumption. As I said before I think the change in ratio from tyres is marginal.
2 Under dia, over revving the motor moves it out of its best rev range
3 Wheel weight, or unsprung weight, has similar effects as bad shocks, worse consumption.
4 Wider tyres, may or may not increase decrees traction, depends on pounds per square inch and actual optimum compound.
5 Wider tyres, increasing the foot print will increase fuel consumption I totally disagree. The foot print will become wider but shorter at the same time meaning it will remain the same. It's a fallacy that wider tyres give more grip or a larger contact patch.
6 Low tyre pressures will increase foot print, increase fuel consumption, see No5 The foot print will increase in length far more than width. The increase in length increases the angle the tread meets the road, increases tread flex and therefore increases heat. All which increase drag.
7 Lower tyre pressures, reduce rolling dia, may or may not affect fuel consumption, but is a mute point since No6 will out weight it in effect.

Best fuel consumption tyres are skinny as hell bald and pumped up to like 200000000 psi Skinny for less wind drag, high pressure and large diameter for minimal angle the surface of the tyre meets the road.

Worse fuel consumption is soft compound, 8ft wide and running 0.00003 psi

The ideal is some where in the middle


Race cars use wider tyres but they gain the advantage of longer life and more durability which means they car run a softer compound and acheive the same tyre life.

Nope sort off. Race cars work on a forulur. Distance, grip weight. = compound with weight. And heat of the day loading etc Yep and then they will run the widest tyre the rule will allow (on road) for the best durability. The formula is to calculate the compound.


In normal road vehicles by fitting wider tyres we actually get less grip because the tyres will heat up less which means the rubber will stay harder and provide less grip but they will last longer. Not mentioning aquaplaning as I think most will be aware of this.

Nope
Forget about heat in road rubber, it’s a small issue. Manufactures spend a lot on R&D to get rid of heat. Wider tyres = less grip only if the foot print is over the ideal pounds per square inch for the compound. So bigger is not always worse. Way to big is just as bad as two small Yes they spend a lot on R&D and while different size tyres of the same model will have a different construction they rarely vary in compound because general public don't worry about stuff like this like we do.

Maybe it's the wind resistance of the larger tyres that causes the extra fuel consumption.

Yes if you have exposed wheels. Like a F1 car. See the elfin? cars of the 70s
You also need to look at the relationship between body and wheels. Big topic minor effect See my previous post and follow the link. A care like the Honda CRX has very little wheel exposure but a 20mm wider tyre changed the CD of the entire vehicle by near 10%.

All this thought has come about because I just like the look of bigger tyres. Not too big, nothing like 37" or anything but about 32" is a good look on a 4x4 just my personal preference and as I said about the bigger size will last longer. But I don't want the extra fuel consumption so if I change my transfer box ratio to maintain the original gearing will my fuel consumption stay the same.

You have a d2 TD5, and you want to run 32” rubber. You are in the NT so mud can be an issue.

Ok assuming you are going to run with a mud. The difference between 29” mud and 32” is minimal. If you were changing to 32 from 29 mud, probably 20-30 klm a tank. If you are changing from 29 road rubber maybe 60 klms a tank If you read any of my posts in previous threads you will find I hate statements of km per tank. It's the most irrelevant measurement of fuel consumption ever quoted.

29 X 3.145 = 91
32 X 3.145 = 100

9% so I would say given the same with, wind drag on the body is the biggest consumer of your fuel Yes, I agree this is likely the case with the 4x4 bricks we drive but wind drag is greater than we might think.



After everything I have read, university test results and similar, I am convinced tread creates a little more drag but it's easily more than offset by the reduced rolling resistance given by the angle the tread meets the road. The advantage of the reduced angle the trad meets the road should even be enough to also offset the extra weight of the tyre (rotational mass) and changing the gearing will maintain my acceleration. The biggest killer is the extra wind drag which means whatever I do going 265 over 245 will most likely use more fuel.

I probably need to star looking at weather it's cheaper to change my crown and pinions when fitting Tru-Tracs or changing Transfer ratio.

HappyDays

[joel0407]

Sorry I missed in the last bit of my last post that for the extra tread drag to be offset the tyre needs to go up in Diameter.

Happy Days

[Slunnie]

Did your fuel consumption get any better or just more consistent?

What size were/are your 33" tyres and are they wider than the 29" tyre? I'm guessing that if they were not that much wider the extra wind drag was offset by the better (less) rolling resistance of the larger diameter tyre.

Happy Days
The fuel consumption was more consistent overall, but economy is better with corrected gearing around town.

The 29's were 235/70-16, the ST's and STT's were 255/85-16, the JT2's were 33x10.5-16.

Personally, I really don't think that wind drag has that significant affect on the consumption. However, I think the varying rolling resistance of tread patterns does. That's something that is notable between changing between like sized tyres but with different tread patterns, but I don't recall off the top of my head being notable when I changed from 255/85-16 Cooper STT to 285/75-16 Goodyear MTR which are comparably open pattern but one wide and one narrow.

[Dougal]

Sorry I missed in the last bit of my last post that for the extra tread drag to be offset the tyre needs to go up in Diameter.

Happy Days

Going up in diameter may decrease rolling resistance by a few percent.

Changing tread pattern can double rolling resistance. One is never going to compensate for the other.